Studio Tests (Full frame)
Again the Tamron 24-70mm F2.8 VC does extremely well in our tests on full frame, and while it's not quite as good as its Canon counterpart wide open at the long end, the results are very similar once stopped down to F4. The barrel distortion at 24mm is pretty strong though, and would probably need correcting when shooting geometric subjects such as architecture.
|Sharpness||At F2.8, central sharpness is high at all focal lengths, but the edges are somewhat softer, especially towards the tele end. Stop down to F4, though, and cross-frame sharpness is impressive through the whole range. As we'd expect optimal results are achieved around F5.6 - F11, beyond which diffraction starts to blur the image.|
|Chromatic Aberration||Lateral chromatic aberration is on the whole very low. You will see a little fringing towards the edges and corners if you go looking for it, but it will rarely have much practical impact.|
|Vignetting||Vignetting is, as usual, much stronger on full frame compared to APS-C. It's very pronounced at wideangle, with a 2 stop drop in brightness in the extreme corners at 24mm and F2.8; this drops progressively on stopping down but never quite goes away. At the telephoto end, there's a precipitous drop in brightness towards the extreme corners of the frame at F2.8, which is likely to be very visible in shots with even-toned backgrounds. However this disappears on stopping down to F4.|
|Distortion||Distortion becomes more pronounced on full frame compared to APS-C. There's rather strong barrel distortion at wideangle (2.5%), changing to visible pincushion distortion at telephoto (-1.7%). This is strong enough that it's likely to need correcting in certain applications such as architectural photography.|
As you'd expect, at minimum focus the story is much the same as we saw on APS-C. The image is distinctly soft at F2.8, but the centre sharpens up well on closing down a stop to F4. The corners are noticeably softer in our flat-field chart test, but sharpen up well on stopping down. Most of the frame is sharp at F11, the with the best overall results at F16. There's visible barrel distortion, and moderate blue/yellow colour fringing in the corners from lateral chromatic aberration.
The Tamron 24-70mm F2.8 VC's standout features is, of course, its built-in 'Vibration Reduction' optical image stabilization system - a feature that neither Canon nor Nikon have managed to include in their counterparts. The mechanism is effectively silent when operational, with only the uncanny stabilization of the viewfinder image betraying the fact that it's running. Unusually, Tamron doesn't make any specific claims about how effective the system might be.
To determine the effectiveness of the VC system we subjected the 24-70mm to our studio image stabilization test, using both the wideangle and telephoto settings. The subject distance for these tests was approximately 2.5m, and the test camera was the Canon EOS 6D using its 'Silent' shutter mode.
We take 10 shots at each shutter speed and visually rate them for sharpness. Shots considered 'sharp' have no visible blur at the pixel level, and are therefore suitable for viewing or printing at the largest sizes, whereas files with 'mild blur' are only slightly soft, and perfectly usable for all but the most critical applications.
|24mm VC OFF||70mm VC OFF|
|24mm VC ON||70mm VC ON|
The 24-70mm, like previous Tamron lenses we've reviewed, performs very impressively in these tests. At 24mm we get almost as good results at 1/2 sec with VC turned on as we do at 1/30 sec with it off - an advantage of about four stops. Even using a shutter speed of 1 sec we get some perfectly sharp shots handheld. Granted that these tests are indoors under ideal conditions, rather than outside in the wind or cold, but even so this is very impressive stuff.
At 70mm the story is much the same: we get essentially as good results at 1/5 sec with VC on as at 1/80 sec with it off, again a clear 4 stop advantage. Things fall apart completely at 0.4sec, but it seems churlish to complain. One slightly unexpected complication is the proportion of not-quite-sharp shots at 1/80 sec and 1/40sec, suggesting the VC unit can't quite compensate fully for the initial impulse of the shutter release. But remember we're looking closely at 20MP images here - for many practical purposes such slightly-blurred shots will be entirely usable.
Real world examples
Our studio tests are designed to give an indication of how effective image stabilization systems can be under ideal conditions. In the real world you may not get quite the same degree of stabilization, especially when shooting under inclement conditions or pointing the camera at awkward angles. As usual you'll normally get best results if you make a habit of waiting a second or so for the system to settle before releasing the shutter.
The examples below should give you an idea of how well the Tamron's VC system behaves in everyday shooting; in both cases the image would be hopelessly blurred without stabilization. They illustrate that VC isn't just useful for taking pictures in lower light.
In the first example we've taken advantage of the ability to shoot hand-held at a slow shutter speed to set the camera to ISO 100 and the lens to F5.6 for optimum results. The shutter speed is 1/15 sec at 70mm on APS-C (112mm equivalent), or three stops slower than we might usually expect to be able to hand-hold, and the the results are perfectly acceptable even when viewing the image at 100%. In the second example we've taken advantage of IS to stop down to F5.6 for depth of field, while still keeping the ISO low.
70mm, Canon EOS 650D
24mm, Canon EOS 650D
|1/15 sec, F5.6, ISO 100||1/8 sec, F5.6, ISO 200|
|100% crop||100% crop|